The present invention relates to a band pass filter, and particularly, to a highly compact band pass filter that has excellent mechanical strength.
In recent years, marked advances in miniaturization of communication terminals, typically mobile phones, has been achieved thanks to miniaturization of the various components incorporated therein. One of the most important components incorporated in a communication terminal is a filter component.
As one type of filter component, Japanese Patent Laid Open No. 2000-68711 and Japanese Patent Laid Open No. 2000-183616, for example, teach band pass filters comprising a dielectric block formed with a plurality of holes whose inner walls are coated with metal plates. As another type of a filter component, band pass filters constituted by forming metal plates on irregular surfaces of a dielectric block are described in xe2x80x9cNovel Dielectric Waveguide Componentsxe2x80x94Microwave Applications of New Ceramic Materials (PROCEEDINGS OF THE IEEE, VOL.79, NO.6, JUNE 1991), p734, FIG. 31.xe2x80x9d
As a need continues to be felt for still further miniaturization of communication terminals such as mobile phones, further miniaturization of filter components, e.g., band pass filters, incorporated therein is also required.
The mechanical strength of the above-mentioned types of filter components is, however, low because holes are formed in, or irregularities are formed on, the dielectric block constituting the main body. Miniaturization of the filter component is therefore impossible. Specifically, in the former type of filter component having holes formed in a dielectric block, mechanical strength of the dielectric block is low around the holes and in the latter type of filter component having irregularities formed on the surface of a dielectric block, mechanical strength is low around the recesses. Therefore, miniaturization of the filter component must be limited to ensure the mechanical strength at such portions.
Thus, in the prior art it is difficult to miniaturize filter components while ensuring sufficient mechanical strength. Therefore, a compact band pass filter that has excellent mechanical strength is desired.
It is therefore an object of the present invention to provide a compact band pass filter having excellent mechanical strength.
The above and other objects of the present invention can be accomplished by a band pass filter comprising a dielectric block of substantially rectangular prismatic shape constituted of a first portion lying between a first cross-section of the dielectric block and a second cross-section of the dielectric block substantially parallel to the first cross-section and second and third portions divided by the first portion and metal plates formed on the surfaces of the dielectric block, thereby enabling the first portion of the dielectric block and the metal plates formed thereon to act as an evanescent waveguide, the second portion of the dielectric block and the metal plates formed thereon to act as a first resonator, and the third portion of the dielectric block and the metal plates formed thereon to act as a second resonator, the metal plates including a capacitive stub formed on a first surface of the dielectric block which is substantially perpendicular to the cross-sections.
According to this aspect of the present invention, a filter function can be obtained even using a rectangular prismatic dielectric block because a desired coupling constant can be produced between the first and second resonators by the capacitive stub formed on the first surface of the dielectric block. Since the band pass filter according to the present invention is constituted of a rectangular prismatic dielectric block, the mechanical strength is extremely high. Thus, even if the overall size of the band pass filter is reduced, sufficient mechanical strength can be ensured.
In a preferred aspect of the present invention, the capacitive stub is formed on at least surfaces of the second and third portions of the dielectric block.
In a further preferred aspect of the present invention, the capacitive stub is further formed on a surface of the first portion of the dielectric block to form a continuous and integral capacitive stub on the surfaces of the first to third portions of the dielectric block.
In a further preferred aspect of the present invention, a portion of the capacitive stub formed on the surface of the second portion of the dielectric block and another portion of the capacitive stub formed on the surface of the third portion of the dielectric block have the same dimensions.
In a further preferred aspect of the present invention, the metal plates further include a first exciting electrode formed on a second surface of the dielectric block which is substantially parallel to the cross-sections and a second exciting electrode formed on a third surface of the dielectric block which is substantially parallel to the cross-sections.
In a further preferred aspect of the present invention, the second and third portions of the dielectric block have the same dimensions.
The above and other objects of the present invention can be also accomplished by a band pass filter comprising:
a first flat resonator and a second flat resonator each having top and bottom surfaces on which metal plates are formed, a shorting surface electrically short-circuiting the metal plates formed on the top and bottom surfaces, a first open surface opposite the shorting surface, a second open surface perpendicular to the shorting surface, and a third open surface opposite the second open surface;
an evanescent waveguide provided between the first and second flat resonators such that the evanescent waveguide is in contact with the entire second open surfaces of the first and second flat resonators;
a first capacitive stub formed on the first open surface of the first flat resonator;
a second capacitive stub formed on the first open surface of the second flat resonator;
a first exciting electrode formed on the third open surface of the first flat resonator; and
a second exciting electrode formed on the third open surface of the second flat resonator.
According to this aspect of the present invention, a band pass filter having no surface irregularities can be obtained because a desired coupling constant can be produced between the first and second flat resonators by the first and second capacitive stubs. Since the band pass filter according to the present invention has no surface irregularities, its mechanical strength is extremely high. Thus, even if the overall size of the band pass filter is reduced, sufficient mechanical strength can be ensured.
In a preferred aspect of the present invention, the band pass filter is substantially a rectangular prism in overall shape.
In a further preferred aspect of the present invention, the first and second flat resonators have the same dimensions.
In a further preferred aspect of the present invention, the first open surfaces of the first and second flat resonators are coplanar.
In a further preferred aspect of the present invention, the metal plates formed on the bottom surfaces of the first and second flat resonators are short-circuited by a metal plate formed on a bottom surface of the evanescent waveguide.
In a further preferred aspect of the present invention, the first and second capacitive stubs are short-circuited by a metal plate formed on a side surface of the evanescent waveguide.
In a further preferred aspect of the present invention, the first capacitive stub is connected to the metal plate formed on the bottom surface of the first flat resonator and the second capacitive stub is connected to the metal plate formed on the bottom surface of the second flat resonator.
In a further preferred aspect of the present invention, the first exciting electrode is formed on the third open surface of the first flat resonator at a portion adjacent to the first open surface of the first flat resonator, the second exciting electrode is formed on the third open surface of the second flat resonator at a portion adjacent to the first open surface of the second flat resonator, the first exciting electrode is prevented from being in contact with the metal plates formed on the top and bottom surfaces of the first flat resonator, and the second exciting electrode is prevented from being in contact with the metal plates formed on the top and bottom surfaces of the second flat resonator.
In a further preferred aspect of the present invention, the first exciting electrode is formed on the third open surface of the first flat resonator at a portion adjacent to the shorting surface of the first flat resonator, the second exciting electrode is formed on the third open surface of the second flat resonator at a portion adjacent to the shorting surface of the second flat resonator, the first exciting electrode is prevented from being in contact with the metal plate formed on the bottom surface of the first flat resonator and is connected to the metal plate formed on the top surface of the first flat resonator, and the second exciting electrode is prevented from being in contact with the metal plate formed on the bottom surface of the second flat resonator and is connected to the metal plate formed on the top surface of the second flat resonator.
The above and other objects of the present invention can be also accomplished by a band pass filter comprising:
a first flat resonator and a second flat resonator each having top and bottom surfaces on which metal plates are formed, a shorting surface electrically short-circuiting the metal plates formed on the top and bottom surfaces, a first open surface opposite the shorting surface, a second open surface perpendicular to the shorting surface, and a third open surface opposite the second open surface;
an evanescent waveguide provided between the first and second flat resonators such that the evanescent waveguide is in contact with the entire second open surfaces of the first and second flat resonators;
a first exciting electrode formed on the third open surface of the first flat resonator; and
a second exciting electrode formed on the third open surface of the second flat resonator,
whereby a first resonation circuit is established between the first exciting electrode and the metal plates, a second resonation circuit is established between the second exciting electrode and the metal plates, and a coupling circuit is established between the first and second resonation circuits,
the band pass filter further comprising means for providing an additional capacitance in parallel with the first resonation circuit and another additional capacitance in parallel with the second resonation circuit.
In a preferred aspect of the present invention, the band pass filter is substantially a rectangular prism in overall shape.